Process for recovery of different weight fractions of oil from shale

ABSTRACT

An oil shale retorting method and apparatus moves a bed of oil bearing shale particles along a generally horizontal path and passes through the bed at spaced points a plurality of discrete nonoxidizing gas streams heated to different oil educting temperatures to vaporize and educe different weight fractions of oil from the kerogen in the shale into the gas streams. Preferably the heated gas streams are also passed through the moving shale bed upstream from distillation of the volatile oil constituents to preheat the shale particles and condense the educted oils which become suspended as stable mists and are mechanically separated from the gas streams. Heat energy is preferably removed from the spent shale particles of the moving bed by burning them with combustion air downstream from the retorting sites, and such heat energy is transferred to the nonoxidizing streams to raise them to different oil educting temperatures before they are passed through the moving bed to educe oil from the shale particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to processes and apparatus for extraction of oilfrom a moving bed of carbonaceous materials such as oil shale.

2. Description of the Prior Art

Systems for retorting a moving bed of oil shale supported on a travelinggrate by passing heat transfer gas streams through the bed to transferheat directly to the shale are disclosed in such U.S. patents as3,325,395 to Ban; 3,560,369 to Rowland et al; 3,644,193 to Weggel et aland 3,475,279 to Bowman. The systems disclosed in such patents, ingeneral, educt oil from the shale at only one temperature and areincapable of: (1) extracting from the shale different weight fractionsof oil unmixed with the combustible products; (2) controlling the typeof oil educted from the shale; (3) removing a pollutant such as hydrogensulphide from the shale; or (4) removing a gaseous chemical feedstocksuch as ethylene from the shale.

The system of the 3,644,193 patent is incapable of separating differentweight fractions of oil from the shale particles. The system of the U.S.Pat. No. 3,560,369 patent requires crushing, grinding and screening ofthe oil shale to segregate the fines; agglomerating the fines with abinder such as heavy oil; and then forming a bed on the traveling gratewith the agglomerated fines occupying a middle layer between upper andlower layers of larger shale particles. The rate of transfer of heatfrom the radiant heaters of U.S. Pat. No. 3,475,279 to the shaleparticles is very low in comparison to a moving bed system wherein aheated gas steam is passed through the moving shale bed, and bothretorting and sintering of the coal or shale particles occur in the samezone in this patent which does not permit close control of thetemperature to which the particles are heated and also results in mixingof the desired volatile products with the combustion gases and crackingof the hydrocarbons. Ambient air rather than a nonoxidizing or neutralgas is passed through the moving shale bed in the second, or sinteringzone of the traveling grate system shown in FIG. 4 of the U.S. Pat. No.3,325,395 and burned with the carbon in the shale particles. Sucharrangement does not permit control of the oil fraction educted and alsomixes the combustion gases with the desired condensable products.

Rotary kiln systems for tumbling and retorting a bed of oil shaleparticles are disclosed in such U.S. patents as 1,383,205; 1,423,716;1,717,786; 2,441,386; 2,664,389; 3,496,094 and 3,844,929, but suchrotary kiln shale retorting systems are incapable of separatingdifferent fractions of oil from the shale and have the disadvantagesthat the shale particles can only occupy a small portion of the kilncross section, e.g., 14 percent, with the result that the tonnagecapacity per unit size of apparatus is relatively low and thatcombustion and retorting occur in the same vessel, which results in lowefficiency and in at least partial cracking of the hydrocarbons withinthe kiln.

SUMMARY OF THE INVENTION

A moving bed oil shale retorting method and apparatus in accordance withthe invention passes through the bed at spaced apart points a pluralityof discrete neutral, i.e., oxygen-free or non-oxidizing gas streamsheated to different temperatures, sufficiently high to vaporize andeduce different gaseous components from the kerogen in the shaleparticles as vapors into the gas streams to concentrate the gaseouscomponents in the discrete gas streams and extract them from the shale.Preferably the nonoxidizing gas streams comprise high heat capacitygases. In the preferred embodiment of the invention the discrete gasstreams are heated to different oil educting temperatures to reducedifferent weight fractions of oil from the shale. The heated gas streamsare also passed through the moving shale bed upstream from distillationof the volatile oil constituents to: (a) preheat the moving bed; and (b)condense the educted oils which become suspended as stable mists and aremechanically separated from the nonoxidizing gas streams. Heat energy isremoved from the spent shale particles by burning them with combustionair downstream from the retorting sites and such heat energy istransferred to the nonoxidizing gas streams to raise them to oileducting temperatures, thereby assuring optimum efficiency.

Preferably the bed of oil shale particles is supported upon a rollergrate comprising a plurality of rotatable apertured horizontal rollerswith elongated stationary filler bars in the nip between adjacentrollers, and the rollers frictionally engage the particles and move themonto and across the filler bars to continually agitate the particles andtransport them through a plurality of retorting zones in whichnonoxidizing heat transfer gas streams heated to different oil eductingtemperatures are passed through the moving shale bed to extractdifferent weight fractions of oil from the particles. High heat capacitynonoxidizing gas streams permit significant reduction in the oileducting temperatures in comparison to relatively low heat capacitygases.

The moving shale bed may also be transported through preheating zonesupstream from the retorting zones wherein the neutral heat transfer gasstreams are passed through the moving bed to condense the oil that wasdriven off in the retorting zones and to preheat the oil bearing shaleparticles before the volatile oil constituents are distilled therefromin the retorting zones. Further, the moving bed of spent shale particlesmay be transported through a combustion zone wherein a stream ofcombustion air is passed through the bed and burned with the carbon inthe spent particles to extract the heat energy therefrom. The heatedcombustion air stream exiting the combustion zone is then passed througha heat exchanger wherein the sensible heat energy is transferred to thenonoxidizing gas streams to raise them to oil educting temperaturesbefore they enter the retorting zones. Preferably the spent shaleparticles are also transported through a cooling zone downstream fromthe combustion zone wherein the combustion air stream is passed throughthe shale bed before it is conveyed to the combustion zone to therebypreheat the combustion air and cool the spent shale particles.

The nonoxidizing gas streams having different weight fractions suspendedtherein as stable mists exiting the preheating zones are sentrespectively to a first separation stage wherein the condensed lighterand less viscous and more hydrogenated oils are removed from the firstnonoxidizing gas stream and to a second separation stage wherein thecondensed heavier, more viscous hydrogen deficient oils are removed fromthe second nonoxidizing gas stream. The type of oil distilled in each ofthe plurality of retorting zones is controlled by the oil eductingtemperatures and the flow rate of the respective oxygen-free gasstreams. After the oil mists are stripped from the nonoxidizing gasstreams in the separation stages, a portion of such nonoxidizing gasesare preferably recycled to the heat exchanger wherein they are reheatedto oil educting temperatures and returned to the retorting zones. Afterthe gases exiting the combustion zone are passed through the heatexchanger, they still contain a significant amount of medium levelsensible heat than can be used, for example, in a waste heat boiler togenerate steam.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows diagrammatically a moving bed oil shale retorting apparatusembodying the invention in which roller grate apparatus for transportingthe bed and agitating the shale particles is illustrated in sideelevation and partially in section; and

FIG. 2 is an enlarged view of several rollers of the FIG. 1 roller grateapparatus with filler bars disposed in the nip there between.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawing, oil bearing raw shale particles 10that have preferably been crushed to a size between 1/4" and 3" aredischarged from a feed hopper 11 onto a plurality of horizontalelongated apertured rotatable cylindrical rollers 12 and stationaryelongated filler bars 14 positioned in the nip between rollers 12 toform a bed 15 of shale particles 10 thereon. Cylindrical rollers 12 arerotated in the same direction so that their cylindrical surfacesfrictionally engage the shale particles 10 and transport them onto andacross the stationary filler bars 14 whose upper surfaces are disposedbelow the upper surfaces of rollers 12 to thereby continuously agitateand tumble particles 10. The moving shale bed 15 may be termed a"dynamic" bed in contrast to a "static" moving bed on a traveling gratewherein the particles do not move relative to the traveling grateplates. Cylindrical rollers 12 may be hollow and have a plurality ofradially extending gas passage apertures, or slots 17 communicating withthe interior thereof. Gas passage apertures 17 may be elongated in adirection parallel to the roller axis or may be in the form ofcircumferential or spiral slots through the cylindrical roller surfaces.Rollers 12 and filler bars 14 form a generally horizontal flat surfacefor shale bed 15 having constantly moving portions which continuouslyagitate and tumble the shale particles 10 and urge bed 15 in a directiontransverse to the axes of rollers 14 at a velocity which is only a minorfraction of the circumferential velocity of rollers 12. The rollers 12and filler bars 14 may constitute a roller grate 18 of the typedisclosed in U.S. Pat. Nos. 4,270,899 and 4,269,593 to Faulkner et alboth of which have the same assignee as this invention.

In the embodiment of the invention illustrated in FIG. 1 roller grate 18transports shale bed 15 from the grate inlet end 19 in sequence througha first preheat zone 20, a second preheat zone 21, a first retortingzone 22, a second retorting zone 23, a combustion zone 24 and a coolingzone 25 and discharges the spent shale particles from the grate exit end26. A gas-confining housing structure 30 encloses roller grate 18 andmoving shale bed 15. A plurality of depending partitions 32 withinhousing structure 30 divide the portion thereof above roller grate 18into a plurality of hood structures 33a-33f, and a plurality of upwardlyinclined bottom wall portions 35 generally in vertical alignment withpartitions 32 divide the portions of housing structure 30 below rollergrate 18 into a plurality of wind box structures 37a-37f disposed belowthe hood structures 33a-33f. Each bottom wall portion 35 may be ofinverted-V cross section as shown or alternatively may have a flat roofsection extending adjacent the lower periphery of one or more rollers12. Hood structure 33a and wind box structure 37a together define firstpreheat zone 20; hood structure 33b and wind box structure 37b togetherdefine second preheat zone 21; hood structure 33c and wind box structure37c together define first retorting zone 22; hood structure 33d and windbox structure 37d together define second retorting zone 23; hoodstructure 33e and wind box structure 37e together define combustion zone24; and hood structure 33f and wind box structure 37f together definecooling zone 25.

Cylindrical rollers 12 may be attached to elongated shafts 38 which passthrough the sidewalls 39 of housing structure 30 and are journalled attheir ends for rotation about their longitudinal axes in nonfrictionbearings 40 outside of housing 30. Suitable seals (not shown) may beprovided between shafts 38 and sidewalls 39 of housing 30. Each rollershaft 38 may have a sprocket (not shown) attached to one end forengagement with a driving chain (not shown). Drive means for a pluralityof rollers are well known such as disclosed on U.S. Pat. No. 3,438,491and are omitted in order to simplify the description.

In first preheating zone 20 the raw shale particles 10 of bed 15 arepartially preheated by the nonoxidizing off-gases from first retortingzone 22 which comprise a first nonoxidizing gas stream and are passedthrough bed 15 within zone 20 as the particles 10 are tumbled by rollers12. The off-gases from first retorting zone 20, which may be at 500° F.,are conveyed to first preheating zone 20 through a conduit 43 connectedat one end to wind box structure 37c and at the opposite end to wind boxstructure 37a and are forced updraft through bed 15 to preheat the shaleparticles 10 and are cooled to condensing temperature. The relativelylight educted oil fraction entrained in the first nonoxidizing gasstream from first retorting zone 22 is condensed as such first gasstream passes through bed 15 and is cooled to condensing temperature infirst preheating zone 20. The oils distilled in first retorting zone 22and condensed in first preheat zone 20 become suspended as a stable oilmist in the first gas stream and are: (a) lighter; (b) less viscous; (c)more hydrogenated; and (d) require less subsequent treatment (i.e.,hydrogenation), than the oils volatized in second retorting zone 23. Thefirst nonoxidizing gas stream having such lighter oil mist entrainedtherein is sent through a conduit 45 which communicates with hoodstructure 33a to a demisting device such as an electrostaticprecipitator which comprises a first oil-gas separator stage 47 andremoves the condensed oils from the first nonoxidizing gas stream.

The fans for forcing the neutral heat transfer gas streams through shalemoving bed 15 and the motors for driving such fans are omitted from thedrawing in order to simplify the description and facilitateunderstanding of the invention.

When oil shale particles 10 are heated, chemical bonds within thecarbonaceous kerogen organic material are broken to release volatileliquid and gaseous products. The term "retorting" denotes thermalconversion of kerogen organic matter to oil vapors and gas, therebyleaving solid particlulate spent shale. Oil shales vary widely in thepercent of kerogen they contain as well as the quantity of magnesium andcalcium carbonates that decompose endothermally upon heating. Ingeneral, western oil shales contain large quantities of magnesiumcarbonates, and their rate of burning must be carefully controlled if itis desired to prevent clinkering and excessive mineral carbonatedecomposition.

When the kerogen is retorted, a normally gaseous fraction, a normallyliquifiable vaporous fraction and an organic residue are formed. Whenthe ratio of hydrogen to carbon in the liquid product is high, thedistilled oil is lighter with a lower density and a lower viscosity. Amore highly hydrogenated oil is desirable because the lighter, lessviscous oil is easier to transport. However, if the lighter oils aresubjected to excessive temperature for long periods of time, they areundesirably cracked to: (1) gases with a higher hydrogen/carbon ratio,and (2) fixed carbon.

The oil educting temperatures to which the first and second nonoxidizinggas streams are heated will vary dependent upon whether eastern orwestern shale is being retorted. In general, the hydrogen/carbon ratioof the products distilled from kerogen is higher in western than ineastern shales, and oils and gases start to be released at a lowertemperature from eastern shales. Consequently the oil eductingtemperature of the first gas stream passed through bed 15 in firstretorting zone 22 is lower when eastern shale is being retorted and alsothe temperature of particles 10 exiting second preheating zone 21 islower.

The oil shale particles 10 of bed 15 transported by rollers 12 fromfirst preheated zone 20 to second preheating zone 21 are further heatedby the nonoxidizing off-gases from second retorting zone 23 whichcomprises a second neutral gas stream and are passed through bed 15within second preheat zone 21 as the shale particles are agitated byrollers 12 to further preheat the oil bearing particles to approximately400° F. and to cool the second gas stream to condensing temperature. Theoff-gases from second retorting zone 23, which may be at 750° F., areconveyed to second preheating zone 21 through a conduit 44 which isconnected at its opposite ends to wind box structures 37b and 37d andhas a joint intermediate its ends with a conduit 42 conveying relativelycool nonoxidizing recycled gases from the first and second oil-gasseparation stages 47 and 48 and which lower the temperature of theoff-gases from second retorting zone 23 before they pass through bed 15in second preheating zone 21. The educted oil in the off-gases or secondnonoxidizing gas stream from second retorting zone 23 is condensed assuch second gas stream is passed updraft through bed 15 in secondpreheating zone 21 to further preheat bed 15 and cool the second gasstream to condensing temperature so that the educted oils becomesuspended as a stable mist therein. The oils volatized, or distilledfrom the oil bearing shale particles 10 in second retorting zone 23 arethe heavier, hydrogen deficient oil fraction, and the secondnonoxidizing gas stream with said heavier oils entrained therein aresent through a conduit 46 which communicates with hood structure 33b tosecond oil-gas separator stage 48 where the heavier, more viscous oilsare removed from the second nonoxidizing gas stream.

The oil shale particles 10 of bed 15 transported by rollers 12 fromsecond preheating zone 21 to first retorting zone 22 may be atapproximately 400° F. Nonoxidizing recycled gases from which oil mist isstripped in first and second separator stages 47 and 48 may be conveyedthrough a conduit 56 to a heat exchanger 50 which heats them to atemperature of approximately 1200° F. at which they enter a conduit 51which communicates with hood structure 33c. Conduit 51 has a joint witha conduit 52 carrying relatively cool recycled gases from the first andsecond separator stages 47 and 48 and which cool the gases conveyedthrough conduit 51 to approximately 850° F. Such cooled gases conveyedby conduit 51 constitute the first nonoxidizing gas stream having atemperature in the range from 600° F. to 900° F. which is passed downdraft through bed 15 in first retorting zone 22 to raise particles 10 toan oil educting temperature of approximately 500° F. which drives off alighter and less viscous fraction of oil from the shale particles.

A conduit 55 which communicates at one end with hood structure 33d has ajoint with conduit 51 and receives and conveys to second retorting zone23 recycled nonoxidizing gases which have been heated to approximately1200° F. in heat exchanger 50 and constitute the second nonoxidizing gasstream. The oil shale particles 10 of bed 15 are transported by rollers12 from first retorting zone 22 to second retorting zone 23 where theyare further heated by the second nonoxidizing gas stream which is passeddowndraft through bed 15 within second retorting zone 23 to raiseparticles 10 of bed 15 to a higher oil educting temperature ofapproximately 900°-1000° F. Such temperature will volatize a heavier,hydrogen deficient oil fraction from the oil bearing particles 10, butthe temperature of the second gas stream is in the range of 900° F. to1200° F. which is below the cracking temperatures of the educted oils,i.e. is not sufficiently high to crack a significant percentage of theoils to gases with a high hydrogen to carbon ratio during the relativelyshort residence time of such gas stream within retorting zone 23. Itwill be appreciated that the type of oil produced is each of a pluralityof retorting zones such as 22 and 23 can be controlled by changing thetemperature of the oil educting gas streams passed through the oil shalebed 15.

As discussed above, the temperatures of the first and secondnonoxidizing gas streams passed through bed 15 in first and secondretorting zones control the type and amount of oil distilled into thegas streams. The first and second nonoxidizing gas streams preferablycomprise high heat capacity gases which permit the oil eductingtemperatures to be significantly reduced in comparison to gas streams ofrelatively low heat capacity gases, thereby reducing the possibility ofcracking of hydrocarbons. In addition to controlling the temperature ofthe first and second oxygen-free gas streams, the flow rate of suchfirst and second oxygen-free gas streams forced through bed 15 in firstand second retorting zones 22 and 23 is varied by changing the pressurecreated by the fans (not shown) which drive such gas streams in order toregulate the quantity and type of oils distilled into the gas stream inthe respective retorting zones. In alternative embodiments having aplurality of retorting zones, a pollutant such as hydrogen sulfide isremoved by controlling the temperature and flow rate of the gas streampassed through the moving shale bed in one retorting zone so that thepollutant will have a higher concentration in the off-gases from suchzone and isolating the off-gases exiting from such zone. In still otherembodiments, a gaseous chemical feedstock such as ethylene is removedfrom the oil shale by controlling the temperature and/or flow rate ofinert gas stream which passes through the moving shale bed in theretorting zone and isolating the off-gases exiting from such zone.

The temperature of particles 10 exiting second preheating zone 21 mayapproach retorting temperature and result in undesirably distilled lightoil from particles 10 into the second gas stream in which the heavieroil fraction is entrained. In order to prevent such mixing of light andheavy oil fractions and maintain the segregation thereof, in alternativeembodiments the second gas stream exiting second retorting zone 23 ispassed through moving bed 15 in first preheating zone 20 and the firstgas stream exiting first retorting zone 22 is passed through moving bed15 in second preheating zone 21.

At the completion of retorting, the spent shale particles 10 of bed 15are transported by rollers 12 to combustion zone 24 where the preheatedcombustion air is burned with the residual carbon in particles 10 tothus extract all heat energy from the spent oil shale particles 10.Ambient air is conveyed to cooling zone 25 through a conduit 58communicating with wind box structure 37f and is passed updraft throughbed 15 within cooling zone 25 to preheat the combustion air stream andalso cool the spent shale particles 10. The preheated combustion airfrom cooling zone 25 is sent to combustion zone 24 through a conduit 60which communicates at its opposite ends with hood structure 33f and withwind box structure 37e. Such preheated combustion air stream is passedupdraft through bed 15 in combustion zone 24 and burned with theresidual fixed carbon in the spent shale particles 10 within combustionzone 24. Combustion zone off-gases may be at approximately 2300° F. andare sent to heat exchanger 50 through a conduit 62 which communicateswith hood structure 33e and are passed through heat exchanger 50 whichmay be of the type disclosed in U.S. Pat. No. 3,644,193 and wherein aportion of the heat is recovered and transferred to the first and secondrecycled nonoxidizing gas streams which are sent to the first and secondretorting zones 22 and 23. After passing through heat exchanger 50 thecombustion zone off-gases still contain a significant amount ofmedium-level heat that can be used in a waste heat boiler (not shown)which could, for example, generate electricity for the oil shaleretorting sytem.

In general, the ratio of hydrogen to carbon of the volatile productsdistilled from the kerogen in western oil shale is higher than ineastern shale and consequently less liquid product is produced and moreresidual carbon remains in the spent particles 10 after retorting ofeastern shales. Because the amount of carbon remaining in the spentshale products is higher, the amount of heat generated by burning thefixed carbon in the particles with combustion air in combustion zone 24will be higher with eastern shales and consequently the flow rate of thecombustion air stream through combustion zone 24 and cooling zone 25will be higher when eastern shale is being retorted than when westernshale is being retorted.

In an alternative embodiment (not shown) the gases exiting cooling zone25 are not passed through combustion zone 24, air at atmospherictemperature is passed through bed 15 in both combustion zone 24 andcooling zone 25 to limit the temperature rise within combustion zone 24,and the gases exiting both zones are combined and sent to heat exchanger50. In this embodiment substoichiometric air is preferably furnished tocombustion zone 24 to provide a high percentage of carbon monoxide inthe exiting gases.

Nonuniform size oil bearing shale particles 10 fed into roller grate 18are continuously agitated as they are transported by rollers 12 onto andacross filler bars 14 and quickly segregate in size with the coarserparticles remaining at the top of bed 15 and the finer material settlingto the bottom of the bed. This is advantageous in first and secondretorting zones 22 and 23 because downdraft heated first and secondnonoxidizing gas streams are passed through bed 15 in such retortingzones and the larger shale particles 10 are exposed to the hottest gasesfor the longest time. The smaller shale particles 10 that are retortedfaster and do not require as high temperature for eduction of oil areadvantageously exposed to a lower average gas temperature. Roller grate18 transports shale particles 10 at the bottom of bed 15 at highervelocity than particles adjacent the top of the bed, and consequentlythe smaller shale particles at the bottom of the bed, which do notrequire as much time to retort as the larger particles, remain in theretorting zones 22 and 23 for shorter times.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of retortingshale particles containing kerogen comprising the steps ofmoving a bedof said particles along a generally horizontal path of travel, passingthrough said bed at spaced apart points along said path of travel aplurality or discrete streams of nonoxidizing gas free of products ofcombustion heated to different temperatures sufficiently high tovaporize and educe different gaseous components from the kerogen in saidparticles as vapors into said gas streams, individually withdrawing saidgas streams along said bed, and separating said different componentsfrom said gas streams.
 2. A method of recovering oil from oil bearingshale particles comprising the steps ofmoving a bed of said particlesalong a generally horizontal path of travel, passing through said bed atspaced apart points along said path of travel first and second streamsof nonoxidizing gas free of products of combustion heated to first andsecond different temperatures sufficiently high to vaporize and educe afirst relatively light and a second relatively heavy fraction of the oilfrom said particles as vapors into said first and second gas streamsrespectively, said first and second temperatures being below thecracking temperatures of said light and heavy fractions, individuallywithdrawing said first and second gas streams, and separating said firstrelatively light and said second relatively heavy oil fractions fromsaid first and second gas streams respectively.
 3. A method ofrecovering oil from oil bearing shale particles comprising the stepsofmoving a bed of said particles along a generally horizontal path oftravel, passing through said bed at a first point a first stream ofnonoxidizing gases free of products of combustion heated to a firsttemperature in the range from 600° F. to 900° F. required to vaporizeand educe a first light fraction of the oil from said particles as avapor into said first gas stream, said first temperatures being belowthe cracking temperature of said first fraction, passing through saidbed, at a second point downstream along said path of travel from saidfirst point, a second stream of nonoxidizing gases free of products ofcombustion heated to a second temperature in the range from 900° F. to1200° F. sufficiently higher than said first temperature to vaporize andeduce a second fraction of the oil heavier than said first fraction as avapor from said particles into said second gas stream, said secondtemperature being below the cracking temperature of said secondfraction, individually withdrawing said first and second gas streamsalong said bed, and separating said first light and said second heavyoil fractions from said first and second gas streams respectively.
 4. Amethod in accordance with claim 2 or 3 and including the step of passingsaid first and second nonoxidizing gas streams through said bed prior tosaid separating step at third and fourth points spaced apart along saidpath of travel and upstream from said first and second points to preheatsaid bed and cool said first and second gas streams to condensingtemperatures to condense the educted oils to a mist in the first andsecond gas streams that can be separated therefrom after they havepassed through said bed at said third and fourth points respectively. 5.A method in accordance with claim 4 and including the steps of passing astream of combustion air through said bed at a fifth point downstreamfrom said first and second points and burning said combustion air withsaid carbon in said oil shale particles to supply heat to saidcombustion air stream, andsubsequently transferring heat from saidheated combustion air stream to said first and to said secondnonoxidizing gas streams to raise them to said first and to said secondoil educing temperatures respectively before they pass through said bedto vaporize and educe oil from said particles at said first and secondpoints.
 6. A method in accordance with claim 5 and including the step ofpassing said stream of combustion air through said bed at a sixth pointdownstream from said fifth point and before said combustion air streamis passed through said bed at said fifth point to thereby preheat saidcombustion air stream and cool said particles after they are burned withsaid combustion air.
 7. A method of recovering oil shale from oilbearing shale particles comprising the steps of transporting a bed ofsaid particles along a generally horizontal path of travel defining adownstream direction and through a first retorting zone and a secondretorting zone in sequence in said downstream direction,passing throughsaid bed in said first retorting zone a first stream of nonoxidizinggases devoid of products of combustion heated to a first temperature inthe range from 600° F. to 900° F. required to vaporize and educe arelatively light fraction of the oil as a vapor from said particles intosaid first gas stream, said first temperature being below the crackingtemperature of said light fraction, passing through said bed in saidsecond retorting zone a second stream of nonoxidizing gases devoid ofproducts of combustion heated to a second temperature higher than saidfirst temperature and in the range from 900° F. to 1200° F. required tovaporize and educe a relatively heavy fraction of the oil as a vaporfrom said particles into said second gas stream, said second temperaturebeing below the cracking temperature of said heavy fraction, andseparating said first light and said second heavy oil fractions fromsaid first and second gas streams respectively.
 8. A method inaccordance with claim 7 wherein said transporting step moves said bed insequence in said downstream direction through a first preheating zoneand a second preheating zone upstream from said first and secondretorting zones, and said method includes passing said first and secondgas streams exiting said first and second retorting zones respectivelythrough said bed in said first preheating zone and through said bed insaid second preheating zone prior to said separating step to preheatsaid bed and cool said first and second gas streams to condensingtemperatures to condense the educted oils to a mist in said first andsecond gas streams that can be separated therefrom after they havepassed through said bed in said first and second preheating zone.
 9. Amethod of recovering oil shale from oil bearing shale particlescomprising the steps of transporting a bed of said particles along agenerally horizontal path of travel defining a downstream direction andthrough a first preheating zone, a second preheating zone, a firstretorting zone, and a second retorting zone in sequence in saiddownstream direction,passing through said bed in said first retortingzone a first stream of nonoxidizing gas devoid of products of combustionheated to a first temperature required to vaporize and educe arelatively light fraction of the oil from said particles as a vapor intosaid first nonoxidizing gas stream, said first temperature being belowthe cracking temperature of said light fraction, passing through saidbed in said second retorting zone a second stream of nonoxidizing gasdevoid of products of combustion heated to a second temperature requiredto vaporize and educe a relatively heavy fraction of the oil from saidparticles as a vapor into said second gas stream, said secondtemperature being below the cracking temperature of said secondfraction, passing said first gas stream exiting said first retortingzone through said bed in said first preheating zone to preheat said bedand cool the first gas stream to condensing temperature to condense sadrelatively light oil to a mist in the first gas stream that can beseparated after the first gas stream has passed through said bed in saidfirst preheating zone, passing said second gas stream exiting saidsecond retorting zone through said bed in said second preheating zone tofurther preheat said bed and cool the second gas stream to condensingtemperature to condense said relatively heavy oil to a mist in thesecond gas stream that can be separated after the second gas stream haspassed through said bed in said second preheating zone, separating saidlight oil fraction from said first gas stream exiting said firstpreheating zone, and separating said heavy oil fraction from said secondgas stream exiting said second preheating zone.
 10. A method inaccordance with claim 9 wherein said transporting step also moves saidbed in sequence through a combustion zone and a cooling zone downstreamfrom said first and second retorting zones, and said method includespassing a stream of combustion air through said bed in said cooling zoneto preheat said combustion air and to cool the spent shaleparticles,passing said stream of combustion air exiting said coolingzone through said bed in said combustion zone and burning saidcombustion air with carbon in said particles within said combustion zoneto supply heat thereto, and transferring heat from said combustion airexiting said combustion zone to said first and second nonoxidizing gasstreams to raise the temperature thereof to said first and to saidsecond oil educing temperature respectively.
 11. A method in accordancewith claim 7 or 9 wherein said steps of passing first and secondnonoxidizing gas streams through said bed in said first and secondretorting zones includes passing recycled gases from which said lightand heavy oil fractions have been removed by said light and heavy oilseparating steps through said bed in said retorting zones.
 12. A methodof recovering oil from oil shale bearing particles comprising the stepsoftransporting a bed of said particles along a generally horizontal pathof travel, educing a plurality of different weight fractions of oil fromsaid particles by passing a plurality of discrete oxygen-free gas steamsdevoid of products of combustion through said bed at spaced apart pointsalong said path of travel which gas streams have been heated todifferent oil educing temperatures below the cracking temperatures ofsaid fractions, heating a stream of combustion air by passing it throughsaid bed downstream from said spaced apart points at which saiddifferent fractions of oil are educed and burning said combustion airwith the carbon in said particles, transferring heat energy from saidheated combustion air stream to said oxygen-free gas streams to raisethe temperatures thereof to said different oil educing temperatures, andseparating said different weight fractions of educted oil from saidoxygen-free gas streams.
 13. A method in accordance with claim 12 andincluding the step of preheating said bed and cooling said oxygen-freegas streams to condensing temperatures to condense said different weightfractions of oil to a mist therein prior to said separating step bypassing said plurality of gas streams through said bed at spaced apartpoints along said path of travel upstream from said spaced apart pointsat which said different fractions are educed.
 14. A method of recoveringoil shale from oil bearing shale particles comprising the stepsoftransporting a bed of said particles along a generally horizontal pathof travel defining a downstream direction and through a first retortingzone, a second retorting zone, a combustion zone, and a cooling zone insequence in said downstream direction, passing a stream of combustionair through said bed in said cooling zone to preheat said combustion airand to cool the spent shale particles, passing the heated combustion airstream exiting said cooling zone through said bed in said combustionzone and burning said combustion air with the carbon in said particlesin said combustion zone to heat said combustion air, transferring heatfrom said heated combustion air exiting said combustion zone to a firstoxygen-free gas stream free of products of combustion to raise thetemperature thereof and passing said heated first gas stream throughsaid bed in said first retorting zone to elevate the temperature of theparticles to an oil educting temperature sufficiently high to drive offrelatively light oil as a gas from particles but below the crackingtemperature of said light oil, transferring heat from said heatedcombustion air exiting said combustion zone to a second oxygen-free gasstream free of products of combustion to raise the temperature thereofabove the temperature to which said first gas stream is heated andpassing said second gas stream through said bed in said second retortingzone to elevate the temperature of the particles to an oil eductingtemperature higher than that in said first retorting zone andsufficiently high to drive off relatively heavy oil as a gas from theparticles but below the cracking temperature of said heavy oil, andseparating said relatively light oil from said first gas stream and saidrelatively heavy oil from said second gas stream.
 15. A method inaccordance with claim 14 wherein said transporting step also moves saidbed in sequence in said downstream direction through a first preheatingzone and a second preheating zone upstream from said first and secondretorting zones, and said method includes passing said first and secondoxygen-free gas streams exiting said first and second retorting zonesrespectively through said bed in said first preheating zones and throughsaid bed in said second preheating zone prior to said separating step topreheat said bed and cool said first and second gas streams tocondensing temperatures to condense the educted oils to a mist in thefirst and second gas streams that can be separated therefrom after theyhave passed through said bed in said first and second preheating zones.